This proposal is a competing renewal of Project #5-R01ES08442, entitled Developmental Toxicity of Methyl Mercury, testing the hypothesis that adverse developmental outcomes in children are associated with exposure to methyl mercury (MeHg) from consumption of a diet high in fish. Since 1989, the Seychelles Child Development Study (SCDS) has been following a cohort of 711 children born to mothers who consumed an average of 12 fishmeals per week during pregnancy. The children are now adolescents. The question of whether exposure to MeHg from fish consumption adversely affects child and adolescent development has aroused deepening public concern. The emphasis on prenatal exposures has left several important scientific questions unresolved. Exposure does not cease at birth, yet we know little about the long-term effects of postnatal dietary exposure to MeHg from fish consumption. Also, almost nothing is known about the contributions to long-term outcomes of combined prenatal and postnatal exposure to MeHg. The current proposal aims to determine the consequences of combined prenatal and postnatal exposure especially for long-term or delayed effects that might appear as the children mature. We will examine functions within three major developmental domains: complex cognition, sensory and neuromotor functions, and social behavioral outcomes in adolescence. The SCDS cohort will be examine once when they are 15 years old and the second when they are 17 years using a combination of psychological and neurological tests and surveys. Prenatal exposure to MeHg was determined by cold vapor atomic absorption from maternal hair collected at enrollment. We will continue to collect child hair at each visit (as has occurred for each of the past five visits) to estimate both average postnatal exposure across ages, and recent exposure. Multiple regression analyses will be used to assess associations between prenatal and postnatal exposure and each endpoint. If we find delayed neurotoxic effects of MeHg in adolescence, substantial or even radical modifications would be required in how we interpret associations between prenatal and postnatal exposure to MeHg and the trajectory of later health consequences.